1. Field of the Invention
This invention relates to a catalyst deterioration-detecting system for detecting deterioration of a catalyst arranged in the exhaust system of an internal combustion engine for purifying exhaust gases emitted from the engine, and more particularly to a catalyst deterioration-detecting system which detects deterioration of the catalyst by the use of an output from an oxygen concentration sensor arranged in the exhaust system.
2. Prior Art
Conventional methods for detecting deterioration of catalysts for purifying exhaust gases from internal combustion engines include a method which comprises providing O2 sensors (oxygen concentration sensors) arranged upstream and downstream of a catalyst arranged in the exhaust system of an internal combustion engine, changing the air-fuel ratio of a mixture supplied to the engine in response to outputs from the O2 sensors while a vehicle in which the engine is installed is traveling under a predetermined traveling condition, measuring a time period (inversion period) elapsed from the time the air-fuel ratio is inverted to the time the output from the O.sub.2 sensor arranged downstream of the catalyst is inverted, and determining whether the catalyst is deteriorated, from the measured time period (e.g. Japanese Provisional Patent Publications (Kokai) Nos. 2-30915, 2-33408, and 2-207159), a method which comprises comparing between an output from the upstream O.sub.2 sensor and an output from the downstream O.sub.2 sensor, such as a method of determining the ratio between the two sensor outputs (Japanese Provisional Patent Publication (Kokai) No. 63-231252), a method of determining the response ratio between the sensor outputs (Japanese Provisional Patent Publication (Kokai) No. 3-57862), and a method of determining the phase difference time between the sensor outputs (Japanese Provisional Patent Publication (Kokai) No. 2-310453).
All these prior art methods contemplate the oxygen storage capacity of the catalyst, and determine deterioration of the catalyst by quantifying the oxygen storage capacity.
After a further study in order to improve the above-mentioned prior art methods, the present inventors have reached the finding that the inversion period of the output from the O2 sensor becomes shorter as an amount of exhaust gases flowing in the exhaust system of the engine, i.e. the flow rate of exhaust gases in the exhaust system, is larger.
However, the above-mentioned prior art methods do not contemplate the influence of the exhaust gas flow rate upon the determination of deterioration of the catalyst using the inversion period, and as a result, provide room for a misjudgment as to deterioration of the catalyst.